Soil organic matter is important for acid buffering and reducing aluminum leaching from acidic forest soils

Elevated anthropogenic acid deposition has accelerated forest soil acidification in southern China. However, the observed responses to increased acid inputs are quite variable among different forest soils, and the reasons remain unclear. To study the causes for the different responses, soil columns taken from a young pine forest (PF) and an old evergreen broadleaved forest (BF) were applied three levels of acid addition treatments for 60 days. We found that the PF soils were quite acid-sensitive, and the high acid treatment significantly decreased soil pH at 0-20 cm soil layer while significantly increased exchangeable H+ at both 0-20 and 20-40 cm soil layers. Both the low and high acid treatments significantly increased exchangeable Na+ at 0-20 cm soil layer and Ca2+ and Mg2+ at 20-40 cm soil layer. Furthermore, the high acid treatment also significantly increased leaching losses of H+, Ca2+, Mg2+ and Al3+. In contrast, the BF soils were able to buffer both the low and high acid treatments without significantly decreasing soil pH and increasing major exchangeable cations including Al3+ and their leaching losses. The significant differences in soil organic matter content were found to be responsible for these different responses, as a result of significantly positive correlation between cation exchange capacity (CEC) and organic carbon in both forest soils. The BF soils had higher organic matter than the PF soils, therefore a greater CEC for consuming H+ and a stronger capacity for binding cations. Our results explained why the PF soil pH decreased from 4.3 to 3.8, while the BF soil pH did not change significantly over the last 16 years, and highlighted the important roles of soil organic matter in acid buffering and retaining of major cations, especially for Al3+ in the very acidic forest soils in southern China.